Optical package with improved fiber alignment fixture

ABSTRACT

An optical package includes a two-piece fiber subassembly which allows for separate axial and radial alignment of the fiber to the active optical device. The subassembly consists of an inner ferrule encasing the fiber and an outer sleeve which holds the ferrule. The ferrule is moved axially within the sleeve to perfect the axial alignment of the fiber and the device. The sleeve includes a number of openings around its circumference to expose the ferrule. Once axial alignment is achieved, the outer sleeve is attached to the ferrule through these openings to fix the axial alignment.

BACKGROUND OF THE INVENTION

The present invention relates to an optical package with an improvedfiber alignment fixture and, more particularly, to a two-piece fiberferrule which provides simplified axial alignment of the fiber to theactive optical device and allows visible inspection of the stability ofthe fiber axial alignment fixation.

An important factor in the reliability of optical transmission systemsis the stability of the various components--transmitters, repeaters, andreceivers. The packages housing these components have been the subjectof much study, especially the aspect of attaching and aligning theoptical fiber to the active component (laser, LED, photodiode, etc.)contained in the package. The stability of these alignments over longperiods of time is one of the most important components of the overallsystem reliability.

This is especially true for laser-based transmitters designed for highfrequency, single mode transmission systems, where relatively smalllateral movements between the laser and the fiber can result in largecoupling losses. For example, a lateral motion of 2 μm can result in a 1dB coupling loss. Since many of these single mode transmitters aredestined for use in remote locations (undersea, for example), the needto maintain precise and accurate alignment is critical.

An exemplary prior art optical package is disclosed in U.S. Pat. No.4,119,363 issued to I. Camlibel et al on Oct. 10, 1978. In the Camlibelet al arrangement, the optical fiber is inserted in the package throughan epoxy-filled tube. Upon solidifying, the epoxy fixes the position ofthe fiber relative to the tube. To achieve alignment between the fiberand the optical device, the tube is manipulated until maximum opticaloutput is attained. The tube is then soldered into place by heating theentire package. An alternative package arrangement is disclosed in U.S.Pat. No. 4,623,220 issued to D. Grabbe et al on Nov. 18, 1986. Grabbe etal use a fiber alignment pedestal which is located inside the packagenext to the optical device. The fiber is fed through an epoxy-filledopening in the head of the pedestal and positioned axially to obtainmaximum output. The epoxy is then cured to fix this axial alignment. Thepedestal itself is located in an epoxy-filled well and is movedvertically and horizontally to maximize output. The epoxy is then curedin the well to maintain the pedestal in place. A problem with both ofthese arrangements is that the type of epoxy, solder, or other holdingmaterial used to fix the alignment must be carefully controlled; forexample, materials with different melting temperatures must be used forthe two separate alignments of Grabbe et al, so that the fiber andoptical device will remain aligned during any further packageprocessing. Additionally, extreme ambient temperature fluctuationsduring the operation of the device may cause the holding material tosoften and thus disrupt the alignment.

As an alternative, U. K. Patent No. 2,124,402A issued to B. A. Eales etal on Feb. 15, 1984 uses laser welding in place of epoxy or solder, tofix the fiber in place. In particular, the optical fiber is encased in ametal tube and positioned on a heat sink next to the optical device. Ametal clamp is placed over the fiber to hold it in position and is laserwelded to both the heat sink and the metal cover around the fiber.Although the laser welds are more stable than the other forms ofattachment previously discussed, the need to perform these laser weldsinside the package and within close proximity of the active device maybe troublesome.

SUMMARY OF THE INVENTION

The present invention relates to an optical package with an improvedfiber alignment fixture and, more particularly, to a two-piece fiberferrule which provides simplified axial alignment of the fiber to theactive optical device and allows visible inspection of the stability ofthe fiber alignment fixation.

The fiber ferrule of the invention consists of an inner cylindrical tubewhich houses the fiber and an outer cylindrical sleeve which has aninner diameter slightly greater than the outer diameter of the tube soas to allow the tube freedom of axial movement within the sleeve. Thesleeve has at least one hole machined completely through the sidewall toexpose the inner tube. Once alignment with the optical device isachieved, the tube and sleeve are joined together through the holes tofix the axial alignment. Laser welds, for example, may be used to formthis attachment. This attachment can then be physically inspected toinsure that it is satisfactory.

BRIEF DESCRIPTION OF THE DRAWING

Referring now to the drawings, where like numerals represent like partsin several views:

FIG. 1 illustrates a cross-sectional view of an exemplary laser packageformed in accordance with the present invention;

FIG. 2 illustrates an exemplary laser subassembly which may be used inconjunction with the package of FIG. 1;

FIG. 3 illustrates an exemplary fiber subassembly which may be used inconjunction with the package of FIG. 1, illustrating in particular thefiber alignment fixture of the invention; and

FIG. 4 illustrates another view of the fiber subassembly of FIG. 3,taken along line 4--4 of FIG. 3.

DETAILED DESCRIPTION

FIG. 1 shows a completed package 10 with a laser 12 mounted on a stud 14and an optical fiber 16 feeding through a ferrule 18 and aligned withlaser 12. The package can be considered as comprising two separatesubassemblies; a laser subassembly 20 and a fiber subassembly 22. Thefocus of this invention is primarily related to the axial alignment ofthese two subassemblies, since the alignment of the two is oftencritical to the reliability of the final package.

FIG. 2 illustrates in detail a portion of an exemplary laser subassembly20. As shown, laser 12 is attached to a mount 24 which is part of stud14. Since the output from a laser is a divergent beam with a divergentangle in the range of, for example, 10° to 50°, a lens 26 is placed inan opening in a platform 28 using a retaining spring 29, where lens 26is used to collimate the beam into a more desirable parallel form. Lens26 is positioned in the z-direction (indicated in the figure) until therequired spacing between laser 12 and lens 26 is achieved. In mostcases, this spacing should be in the range of 10-20 μm. Active alignmentis then performed to position the axis of lens 26 relative to the outputfrom laser 12. In one active alignment procedure, a video system is usedwhere the optical axis of the lens is first aligned with a fiduciary ona video screen. Laser 12 is then activated, and the emission throughlens 26 is viewed on the screen at a relatively far distance (>150 mm).The position of platform 28 is then adjusted until the emission alignswith the fiducial marking. Once alignment is achieved, platform 28 isattached to mount section 24 of stud 14. In order to achieve highreliability and insure that the relative positions of laser 12 and lens26 remain constant, a series of laser welds are used to attach platform28 to mount 24. Three such laser welds are illustrated at points A, Band C in FIG. 2. A similar set of welds is used to attach the oppositeside of platform 28 to mount 24 (now shown).

Referring back to FIG. 1, the remainder of laser subassembly 20 will bedescribed. A first graded-index (GRIN) lens 32 is positioned in front oflens 26 and is used to focus the output from laser 12 to a small spotsize. First GRIN lens 32 is held in a first retainer 34 which mates withthe housing surrounding laser 12 (shown in FIG. 1). A second GRIN lens36 is positioned at the rear of laser 12, and is used to focus theoutput from the rear face of laser 12, where this light output is usedto monitor the operation of laser 12. A second retainer 38 is used tohold GRIN lens 36 in place.

An exemplary fiber subassembly 22 is illustrated in detail in FIG. 3.Ferrule 18, holding fiber 16, is positioned in a z-direction adjustmentsleeve 40. FIG. 4 illustrates a section of subassembly 22 taken alongline 4-4 of FIG. 3. Fiber ferrule 18 may be axially moved within sleeve40 (as indicated by the arrows), since the inner diameter (ID) of sleeve40 is chosen to be slightly greater than the outer diamter (OD) offerrule 18. It has been found that a gap between the two pieces of0.001-0.002 inches is sufficient for this purpose. Once the relativepositions of ferrule 18 and sleeve 40 are fixed, the attachment isachieved by filling in this gap at a number of locations between the twomembers, such as by laser welding. During the attachment process, theradial (x,y) position of ferrule 18 within sleeve 40 may shift, due tothe induced stresses between the pieces. Thus, as discussed above, it isimportant that the axial alignment be performed prior to any radialadjustment.

The z-alignment is achieved by bringing face 42 of first GRIN retainer34 into flush contact with face 44 of z-sleeve 40, as shown in FIG. 4.Laser 12 is then activated and ferrule 18 is moved in the x,y,zdirection until the output through fiber 16 is optimized. A series oflaser welds, or other suitable form of attachment, is then used to fixthe position of ferule 18 in sleeve 40 by filling the gap betweenferrule 18 and sleeve 40. This attachment is advantageously performed inthe present invention by virtue of the design of z-sleeve 40. As seen byreference to FIGS. 3 and 4, sleeve 40 includes a number of openings, orslots 46, which are machined completely through sleeve 40. Thus,portions of ferrule 18 are visible through openings 46. Laser spot weldsmay then be performed in the gap through openings 46 to join sleeve 40to ferrule 18. In this process, a slightly defocused laser beam is usedto melt material from ferrule 18 and combine it with melted materialfrom sleeve 40 to fill the gap. Since the locations of these welds arevisible, any welds of inferior quality can be seen immediately andredone. This is in contract to some prior art arrangements which used"blind welds" that could not be checked during the welding process.

In one embodiment of the invention, four such openings 46, spaced 90°apart, are formed around the circumference of sleeve 40. This is thearrangement illustrated in FIG. 4. A series of overlapping spot welds,for example, six welds, may be performed in each opening 46 to form acontinuous fillet of attachment material in the gap between ferrule 18and sleeve 40. The symmetry of welds is preferred for stability and themultiplicity is preferred for strength.

At the completion of this welding operation, the radial, orx,y-direction alignment of sleeve 40 to retainer 34 may proceed.Although faces 44 and 42 of retainer 34 and sleeve 40 may be perfectlyaligned in the x,y-direction at the completion of the axial alignmentprocess, this is highly unlikely. Thus, some type of radial alignmentwill be required. In most cases, these two pieces may be formed tocomprise the same or nearly the same, outer diameter. This situation isnecessary to achieve a reliable radial alignment. In the prior art, astraightforward active alignment, involving the movement of the piecesin the x and y directions until the light output is optimized, wasutilized.

An improved x,y alignment process has been developed for use with thislaser package, and can in general be used with any radial alignment oftwo cylindrical pieces having similar outer diameters. Our copendingapplication Ser. No. 061,629 discloses the novel radial alignmentprocedure which may be employed with this package.

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments which can bedevised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention. In particular, although the foregoingillustrative embodiment described the packaging of a junction laser, itis apparent, of course, that similar principles apply to packages forother optical sources (such as LEDs) as well as optical detectors (forexample, avalanche photodiodes). Additionally, other materials, such asepoxy or solder, may be used to fix the relative positions of theferrule and sleeve through the openings in the sleeve.

What is claimed is:
 1. An optical package comprising:an opticalsubassembly including an active optical device located on a mountingstructure, and focusing means located between the optical device and theexterior of the package; and a fiber subassembly including an opticalfiber encased in a ferrule, the ferrule being located within acylindrical sleeve and capable of axial motion within said sleeve toachieve optimum axial alignment between the optical subassembly and thefiber subassembly, said sleeve including at least one openingtherethrough to expose said ferrule to allow attachment of said ferruleto said sleeve.
 2. An optical package as defined in claim 1 whereinlaser welds are used to attach the ferrule to the sleeve.
 3. An opticalpackage as defined in claim 1 wherein a plurality of openings are formedin the sleeve to allow for a plurality of attachment sites of theferrule to said sleeve.
 4. An optical package as defined in claim 3wherein laser welds are used in each opening of the plurality ofopenings to attach the sleeve to the ferrule.
 5. An optical package asdefined in claim 4 wherein a plurality of laser welds are formed in eachopening of the plurality of openings.
 6. An optical package as definedin claim 3 wherein a plurality of four symmetrically disposed openingsare formed in the sleeve.
 7. An optical package as defined in claim 1wherein the active optical device comprises a semiconductor laser whichis positioned on a mount in the optical subassembly.
 8. An optical fibersubassembly comprising:a cylindrical ferrule for encasing an opticalfiber; and A cylindrical outer sleeve surrounding the ferrule, the innerdiameter of said sleeve being slightly greater than the outer diameterof said ferrule so as to allow said ferrule the ability to move in anaxial direction within said sleeve, said sleeve including at least oneopening completely therethrough to expose said ferrule, the ferrule andsleeve being permanently joined through said at least one opening uponachievement of axial alignment of the fiber subassembly with anassociated optical device.
 9. The optical fiber subassembly of claim 8wherein laser welds are used to join the sleeve to the ferrule.
 10. Theoptical fiber subassembly of claim 8 wherein a plurality of openings areformed in the sleeve.
 11. The optical fiber subassembly of claim 10wherein a plurality of four, symmetrically disposed openings are formedaround the circumference of the sleeve.
 12. The optical fibersubassembly of claim 11 wherein a series of laser welds are formed ineach opening of the sleeve.
 13. The optical fiber subassembly of claim 8wherein a gap in the range of 0.001-0.002 inches exists between theferrule and the sleeve.
 14. A method of achieving axial alignmentbetween an optical device mounted on a subassembly and a fiber encasedin a ferrule surrounded by an outer sleeve member, the sleeve memberincluding at least one opening through its sidewall so as to expose theferrule, the method including the steps of:a. activating the opticaldevice b. viewing the output from the optical device through the fiber;c. moving the ferrule axially within the sleeve until optimum lightoutput is achieved; and d. attaching the ferrule to the sleeve in theoptimum position through the at least one opening in the sleeve.
 15. Themethod of claim 14, wherein in performing step (d), laser welding isused to attach the sleeve to the ferrule.
 16. The method of claim 14,wherein the sleeve includes a plurality of symmetrically disposedopenings such that in performing step (d), a series of attachments areperformed in each opening.
 17. The method of claim 16 wherein inperforming step (d), laser welding is used to form each attachment ofthe plurality of attachments.